Advice on how to understand UV unwrap a rounded object
Stretching

First of all, you should make sure that your UV map is free from stretching as much as possible. It is impossible to make it complete stretch free unless you split every triangle. If you plan to draw on the texture only in the 3D editor, that might be a possibility.

To check stretching, assign a checkered texture, like the built in in blender.

Also, in the UV/image editor, in the left panel (hit N), under display, you can check the stretch checkbox. It will color code stretching. Pure blue means no stretching. Various nuances of light blue, cyan, green, and all the way to red means more or less stretching.
There are two kinds of stretching to select here, angle and scale. I mostly use the angle stretching.

Cylindrical mapping

To make it easy to paint latitude and longitude lines, you can use cylindrical mapping. In the 3D editor, in edit mode press U and select Cylindrical projection. The projection is relative your view. Press 5 and 1 on the numpad before doing the projection.

However, this technique will give you serious stretching at the poles. The top row and bottom row in the texture will be be concentrated to a single vertex.